Move PostgreSQL into Docker

[zigouras]

Describe the task

The backend setup requires a native installation of Postgres for the vacantlotsdb, e.g. on your Mac, Windows or Linux workstation. We already use Docker to house the Python environment that runs the data load script. Docker easily supports Postgres. We should move Postgres into Docker which will greatly simplify our backend installation and development environment by removing the need for developers to set up their own Postgres.